Colloidal particles are removed using coagulating chemicals, such as, alum and ferric salts. The chemical coagulants neutralize the electrical charges of the particles that cause them to clump together. These coagulants form metal hydroxides that can absorb on particles together to form a floc.
Flocculation is the agglomeration of destabilized particles into microfloc, and later into bulky floccules that can be settled called floc. The introduction of another reagent, called a flocculant or a coagulant aid may promote the formation of the floc. The flocculation process provides contact between particles to promote their gathering together into a floc and removal by sedimentation and filtration.
Inorganic polymers (activated silica) and natural polymers (starches, alginate) were the first flocculant to be used. Later synthetic polymers became popular due to ease of use.
Activated silica (AS) was the first flocculant used in water treatment. It gives good results, especially when used together with alum in cold water. It is generally added after the coagulant and is prepared immediately before use by partially neutralizing the alkalinity of a solution of sodium silicate. Activated silica is prepared in alkaline conditions [1].
When producing activated silica, it is necessary to consider a large number of parameters affecting the characteristics of the final product, such as the activating agent, the concentration of various reactants, pH, reaction time and temperature, and the way the reagents are mixed [2]. Attempts have long been made to isolate the various chemical reactions involved in the formation of activated silica and their effect on the final product. Baylis, J. R, U.S. Pat. No. 2,217,466, for instance, discloses forming activated silica by partial neutralization of alkali metal silicate by the addition of a N/50 of sulfuric acid to a 1 to 3 percent silicate solution. The aging concentration of silica in the mixture is about 1.5% and the final concentration before use is 1% by dilution with water. About 85% of the sodium silicate in the batch preparation is neutralized by the acid and requires an aging time of about one hour before use. This method of making activated silica is difficult to accomplish due to gelling and long aging time. This method suffers from the disadvantage that the making of the activated silica requires close control of alkalinity for best results.
Schworm et al., U.S. Pat. No. 2,234,285, tried to use sulfate salts, such as aluminum and iron sulfates instead of sulfuric acid to partially neutralize sodium silicate. The mixture is added into the water without aging the activated silica. Baker et al., U.S. Pat. No. 2,310,009 improved the use of metal salts by aging the activated silica to incipient gel formation and then diluting it with water to stabilize the activated silica. This method suffers from the disadvantage that the making of the activated silica requires higher reagent cost.
Hay et al., U.S. Pat. No. 2,444,774, tried to use ammonium sulfate to make activated silica with the advantage that the product is not prone to gelling. This may be advantageous to water treatment plants that use chloramines as the primary disinfectant. This method suffers from the disadvantages of expensive reagents and ammonia added to the water may not be wanted in the finish product.
Walker, J. D., U.S. Pat. No. 2,769,785, tried to use chlorine to make activated silica that lends to continuous type of operation. This method suffers from the disadvantages of complex apparatus and control of the activated silica making process. This process is complicated to implement.
Elston, J. W., U.S. Pat. No. 2,466,842, Mahler, W., U.S. Pat. No. 4,213,950, and Arika et al., U.S. Pat. No. 4,554,211, demonstrated the production of silica gel that is unsuitable as a flocculant in the water treatment process.
Rushmere U.S. Pat. No. 4,954,220 and U.S. Pat. No. 5,176,891 showed the benefits of using different activating agents for the production of activated silica use in papermaking retention and drainage.
Moffett et al., U.S. Pat. No. 5,980,836, No. 5,853,616, and No. 5,648,055 showed the preparation of polyaluminosilicate microgels by using an activator such as alumina, alkali metal aluminate, and other aluminum salts which are more expensive than the sulfuric acid used in this invention to make the activated silica. The product polyaluminosilicate microgels is unstable and not suitable for the removal of natural organic matter (NOM) in water.
Haase et al., U.S. Pat. No. 5,069,893 showed the preparation of a polymeric basic aluminum silicate-sulfate used for turbidity removal in water treatment.
Lind et al., U.S. Pat. No. 5,573,674 showed the preparation of an activated sol using sodium aluminum sulfate as the activating agent. The sol is prepared in a basic condition.
The most common activating agents are: sulfuric acid, alum, chlorine, sodium bicarbonate, carbon dioxide, aluminum sulfate, sodium aluminum sulfate, and sodium aluminate. Among these agents sulfuric acid is the cheapest.
The applications of activated silica depend mainly on the size, charge, and shape of the polymer. During the aging period the monomer, dimer, or low molecular weight polymer of silicic acid formed on neutralization of the silicate alkalinity by acidic material increases in size. The mixture gels if the aging process is not stopped by dilution, addition of alkali or other means. The size of the activated silica polymer can be varied over a wide range by controlling the aging time. The charge of the polymer may be varied by changing the pH or by forming the polymer in the presence of ions and molecules that are adsorbed. This changes the chemical/physical properties of the polymer.
In summary, prior methods of making activated silica called for:                A close control of alkalinity, such as the Baylis method;        The use of relatively expensive activating reagent, such as sulfate salts;        The use of gases such as carbon dioxide, chlorine, and sulfur dioxide that can cause asphyxiation; or        Relatively long aging time.        
These prior methods had the following disadvantages:                Requires close monitoring of the preparation process;        Activated silica pH is above 7;        Frequent maintenance due to failed or gelled silica;        Close monitoring for gas (chlorine, carbon dioxide, sulfur dioxide) leaks;        Long aging time; or        Poor quality control in the manufacture of activated silica.        
Thus, there is a need for a fast, safe, low cost, and efficient process for making a stable acidified activated silica solutions. My invention fills that need.